<!doctype html>
<html>
	<head>
		<style>
			body{background:red}
			div{background:blue;
				width:300px;
				height:300px;
				opacity:0.5;
				position:absolute;
				top:0;
				left:0
			}
		</style>
	</head>
	<body>
		看到我你就成功了
		<div></div>
	</body>
</html>